Please let me address the squish clearance as mentioned by Zagnut and Downunder. The squish clearance has a huge influence that is not readily apparent. It's affects are often confused with changes in compression. The whole point of having a squish band is to promote turbulence in the combustion chamber. How do you quantify turbulence? By calculating how fast the air/fuel mixture is "squished" out from between the head and piston. This is done easily with computer software from TSR software.
http://www.tsrsoftware.com/
Changing the squish clearance on a model airplane or car engine that is set up "tight" to begin with (less than .014 inch) by just a couple of thousandths will make a huge difference in average squish velocity as compared to compression ratio. Using results I got from TSR's squish velocity calculating software I machined diffferent heads for some of my engines and noted very positive results. I was able to keep compression very high, had great idle and throttle response and had better peak power. The motorcycle guys have known about squish properties for years. If your squish is too tight for your engine's operating range the symptoms that crop up are rough running at high rpm and even pre-ignition or detonation. If the squish is too "lazy" you will have an engine that feels a little flatter but will tolerate a slightly leaner mixture setting and how you set the needle will be less critical. We are talking differences the thickness of a sheet of notebook paper! These kind of changes have EXTREMELY small affects on the actual compression ratio. Do the math and you'll see.